The present invention relates to analogue to digital conversion and to clamping signals, that is, ensuring that a particular part of a signal is held at a specified value.
The invention is applicable in a telecine projector, for example. A telecine projector is a device which converts images stored in the frames of movie film into television signals for display, recordal or transmission. The film images are projected onto an array of photoelectric sensors to generate an electrical signal which is subsequently processed to meet the requirements of a chosen television signal standard. It is desirable that at least some of the signal processing takes places digitally.
One function of the signal processing normally carried out in telecine equipment and television cameras, is known as gamma correction. Another is aperture correction. Gamma correction is applied to the signal to compensate for the non-linearity of the light output with respect to signal input of cathode ray tubes (CRTs), which will eventually display the signal. The light output is normally related to the signal input magnitude by a power law, typically with an index of about 2.5. Compensation is provided in the telecine projector by processing the output of the sensor array, which is assumed to have a linear response, according to a power law with an index which is approximately the reciprocal of the CRT response index, so that the light output of the CRT becomes linearly related to the light input to the sensor array. The index is typically 0.4, but in some circumstances may be as low as 0.2.
If gamma and aperture correction are carried out by analogue circuits, subsequent processing being digital, analogue to digital conversion with 8-bit resolution is found to give satisfactory performance. 8-bit analogue to digital converters (ADCs) capable of operation at up to 50 MHz are commercially available. This frequency is adequately high.
If digital circuits are also used to perform the gamma and aperture correction, 14-bit resolution is found to be necessary, with a sampling rate of at least 30 MHz. At the present time, a 14-bit ADC operable at this rate is not commercially available.
The problem can be overcome by using an ACD as shown in FIG. 1. This type of ADC is known, and is called herein an "iterative ADC". The interative ADC 10 receives an analogue input signal at 12. The analogue signal converted by an 8-bit ADC 14 to provide an output which, after a delay provided by a delay circuit 16, forms one input to an adder circuit 18.
The output of the ADC 14 is also converted back to an analogue signal by a digital to analogue circuit (DAC) 20, and compared by a comparator 22 with the original input signal at 12, which has been delayed by a delay circuit 24. The output from comparator 22 is a difference signal representing the difference between the actual input signal level at 12 and the level represented by the output of the ADC 14. This difference signal is amplified by a factor of 64 by an amplifier 25, and encoded by a further 8-bit ADC 26 to provide the second input to the adder 18. The amplification applied by the amplifier 25 gives the bits of this signal lower significance (by six binary positions) than the bits of the signal from the delay circuit 16. Thus, the adder 18 receives an 8-bit most significant byte from the delay circuit 16 and an 8-bit least significant byte from the ADC 26, the bytes overlapping by two binary places. These inputs ae combined to form a 14-bit output at 28. The overlap in the bits of the bytes allows the least significant byte to take positive or negative values.
When an iterative ADC is used to provide analogue to digital conversion in a telecine projector, before gamma correction, a problem is found to arise. At and near the black level of a signal, the signal level (but not necessarily the voltage level) is zero and small, respectively, and the gradient of the gamma correction power law is extremely high. It is found that if a change in the most significant byte takes place in this region of high gradient, small discrepancies in the gain of the amplifier 25 produce visible and detrimental effects in the final image. The flaws arise because the amplifier 25 will never, in practice, be perfectly linear in its response to have precisely the required gain, so that a small change in the input signal about a threshold of the ADC 14 will not be accurately reflected in the output signal at 28. Discontinuities and discrepancies in the output 28 are amplified by the steep gamma correction characteristic, and hence cause the visible flaws.
It is an object of the present invention to reduce subjective degradation of the final image caused by operation of the iterative ADC.